1. Technical Field of the Invention
The embodiments of the invention relate to integrated circuits and, more particularly, to an output stage of an amplifier.
2. Description of Related Art
In the design of various integrated circuit components, the choice of selecting a particular characteristic for the component depends on many factors. For example, when an amplifier is designed for an integrated circuit, the components (such as transistors) comprising the amplifier have properties designed into them based on desired characteristics for the integrated circuit. One of these electrical properties is the voltage selected to power the components of the amplifier.
Since today's integrated circuits are designed with power consumption in mind, many transistor circuits are now designed to utilize lower supply (also referred to as rail) voltages. The lower supply voltage allows the integrated circuit to consume much less power. The signal levels may be correspondingly smaller as well in relation to the supply voltage. In many instances it may be desirable that the transistors drop as little voltage as possible to reduce power consumption. Accordingly, in a situation where a lower supply voltage is being utilized, it may be desirable for the transistors to have low voltage drop across the junctions. That is, the transistors may be designed to have a low threshold voltage (VT) to operate in the lower rail voltage environment.
Generally, different VT devices are optimized for different voltage ranges. High VT devices may be used in circuits, such as chip I/O (input/output), and tolerate higher voltages. Low VT devices may be used for high-speed lower voltage (and lower power and area) applications in the internal circuitry of the chip. Therefore, low voltage devices with low VT are desirable for analog applications, because the supply voltage may be less and less power may be consumed. However, the use of low VT device may make some circuits difficult to implement.
For example, in an amplifier where the transistors are operating in saturation (where VDS>VGS−VT), there is a minimum value for VDS for the circuit to operate properly. Where multiple transistors are involved, the VGS−VT value may need to have a higher value over all process corners. Also, to keep current flowing at the output stage, the gate-to-source voltage VGS should be above a VT. Combining these factors, it is difficult to achieve this over all process corners with a thin gate, low VT device. One solution is to implement an intermediate level shifting stage(s). However, this level shifting stage at the output is a limiting factor in reducing supply voltage.